Resistance spot welding method and weld member production method

ABSTRACT

A resistance spot welding method comprises: performing test welding; and performing actual welding after the test welding, wherein in main current passage in the actual welding, adaptive control welding is performed, and in subsequent current passage in the actual welding, current passage is performed by constant current control with a current determined based on an electrical property between electrodes in each of main current passage in the test welding and the main current passage in the actual welding.

TECHNICAL FIELD

The present disclosure relates to a resistance spot welding method.

The present disclosure is particularly intended to ensure a stablenugget diameter and improve joint strength in a sheet combinationincluding one or more high strength steel sheets even in the case wherethe effect of a disturbance such as current shunting or a sheet gap issignificant.

BACKGROUND

Overlapping steel sheets are typically joined by resistance spot weldingwhich is one type of lap resistance welding.

Resistance spot welding is a method of squeezing two or more overlappingsteel sheets by a pair of electrodes from above and below and, whileapplying an electrode force, passing a high welding current between theupper and lower electrodes for a short time to join the steel sheets.Heat generated from the resistance to the flow of the high weldingcurrent is used to obtain a spot weld. The spot weld is called a nugget,and results from the overlapping steel sheets melting and solidifying attheir contact portion when the current flows through the steel sheets.The steel sheets are spot-joined by this nugget.

For good weld quality, it is important to form the nugget with adiameter in an appropriate range. The nugget diameter depends on thewelding condition such as welding current, welding time, electrodeshape, and electrode force. To achieve an appropriate nugget diameter,the welding condition needs to be set appropriately according to theparts-to-be-welded condition such as the material, the sheet thickness,and the number of overlapping sheets of the parts to be welded.

In vehicle manufacturing, for example, spot welding is performed atseveral thousand points per vehicle, and parts to be welded (workpieces)conveyed one after another need to be welded. If the state of the partsto be welded such as the material, the sheet thickness, and the numberof overlapping sheets of the parts to be welded is the same at eachwelding location, the same welding condition such as welding current,welding time, and electrode force can be used to obtain the same nuggetdiameter. In continuous welding, however, the contact surfaces of theelectrodes with the parts to be welded wear gradually and the contactareas widen gradually as compared with the initial state. When the samewelding current as in the initial state is passed in such a state inwhich the contact areas have widened, the current density in the partsto be welded decreases and the temperature rise of the weld decreases,resulting in a decrease in nugget diameter. Accordingly, the electrodesare dressed or replaced every several hundred to several thousandwelding points, to prevent the electrode tip diameter from increasingexcessively.

A resistance welding device having a function (stepper function) ofincreasing the welding current after a predetermined number of weldingoperations to compensate for a decrease in current density associatedwith electrode wear has been conventionally used, too. To use thestepper function, the above-mentioned welding current change patternneeds to be set appropriately beforehand. However, considerable time andcost are required to derive appropriate welding current change patternscorresponding to numerous welding conditions and parts-to-be-weldedconditions through tests and the like. Besides, since the state ofprogress of electrode wear varies in actual work, the welding currentchange pattern set beforehand may not always be appropriate.

In addition, in the case where there is a disturbance during welding,such as when a point that has already been welded (hereafter alsoreferred to as “existing weld”) is present near the current weldingpoint or when the parts to be welded have considerable surface roughnessand a contact point of the parts to be welded is present near thewelding point, part of the current is shunted into such existing weld orcontact point during welding. In this state, even when welding isperformed under a predetermined condition, the current density at theposition to be welded which is directly above and below the electrodesdecreases, and so a nugget of a required diameter cannot be obtained. Tocompensate for such an insufficient amount of heat generated and obtaina nugget of a required diameter, a high welding current needs to be setbeforehand.

Moreover, in the case where the surroundings of the welding point arestrongly restrained due to surface roughness, member shape, etc. or inthe case where foreign matter is present between the steel sheets aroundthe welding point, a larger sheet gap between the steel sheets causes asmaller contact diameter of the steel sheets, which may facilitateexpulsion (splash).

The following techniques have been proposed to solve the problems statedabove.

For example, JP 2003-236674 A (PTL 1) discloses a spot welding methodfor high tensile strength steel sheets whereby spot welding is performedthrough the following steps to suppress expulsion caused by poor fitnessin an initial stage of current passage: a first step of graduallyincreasing the current to high tensile strength steel sheets to form anugget; a second step of decreasing the current after the first step;and a third step of, after the second step, increasing the current toperform actual welding and gradually decreasing the current.

JP 2006-43731 A (PTL 2) discloses a current control method in spotwelding whereby such a current that can suppress spattering ismaintained for a predetermined time in an initial part of welding timeto soften the surfaces of parts to be welded and then a high current ismaintained for a predetermined time to grow a nugget while suppressingspattering.

JP H9-216071 A (PTL 3) discloses a control unit of a resistance welderthat compares an estimated temperature distribution of a weld and atarget nugget and controls the output of the welder to obtain the setnugget diameter.

JP H10-94883 A (PTL 4) discloses a welding condition control method fora resistance welder of detecting the welding current and the voltagebetween tips, simulating a weld through heat transfer calculation, andestimating the nugget formation state in the weld during welding toachieve good welding.

JP H11-33743 A (PTL 5) discloses a resistance welding system thatcalculates, from the sheet thickness of parts to be welded and thewelding time, the cumulative amount of heat generated per unit volumewith which good welding of the parts to be welded is possible, andadjusts the welding current or voltage to generate the calculated amountof heat per unit volume and per unit time, to achieve good weldingregardless of the type of the parts to be welded or the wear state ofthe electrodes.

CITATION LIST Patent Literatures

-   PTL 1: JP 2003-236674 A-   PTL 2: JP 2006-43731 A-   PTL 3: JP H9-216071 A-   PTL 4: JP H10-94883 A-   PTL 5: JP H11-33743 A

SUMMARY Technical Problem

However, with the techniques described in PTL 1 and PTL 2, given that anappropriate welding condition varies depending on the presence orabsence of a disturbance and the magnitude of the disturbance, a desirednugget diameter cannot be ensured without expulsion when a larger sheetgap or current shunting than expected occurs.

The techniques described in PTL 3 and PTL 4 need complex calculation toestimate the nugget temperature based on a heat transfer model (heattransfer simulation) and the like. This requires a welding control unitthat is not only complex in structure but also expensive.

Moreover, the techniques described in PTL 1 to PTL 5 are not concernedwith a method of improving joint strength in welding of a sheetcombination including one or more high strength steel sheets.

It could therefore be helpful to provide a resistance spot weldingmethod that can obtain a nugget of an appropriate diameter and alsoimprove joint strength (hereafter also referred to as “joint strength ofhigh strength steel sheets”) in welding of a sheet combination includingone or more high strength steel sheets (specifically, one or more steelsheets having a tensile strength of 590 MPa or more, and particularly atensile strength of 980 MPa or more), regardless of whether there is adisturbance.

It could also be helpful to provide a weld member production method ofjoining a plurality of overlapping metal sheets by the resistance spotwelding method.

Solution to Problem

We conducted intensive study to achieve the object stated above, anddiscovered the following:

If there is a disturbance such as current shunting or a sheet gap, theobtained nugget diameter varies even when welding is performed byconstant current control under the same condition as in the case wherethere is no disturbance, as mentioned above.

By performing test welding beforehand and then performing actual weldingby adaptive control welding that controls the current passage amount(the current and the voltage between electrodes) with the cumulativeamount of heat generated that is obtained by the test welding being setas the target, appropriate current passage can be performed with theeffect of a disturbance being taken into consideration. Consequently, acertain nugget diameter can be obtained regardless of a disturbance.

Moreover, an effective way of ensuring necessary joint strength in asheet combination including one or more high strength steel sheets is toperform subsequent current passage for heat treatment of a weld aftermain current passage for nugget formation.

However, if the subsequent current passage is performed by adaptivecontrol in the presence of a disturbance, the current densitydistribution of the weld and thus the heat generation pattern change dueto the disturbance in some cases, making it impossible to achieve thepredetermined heat treatment effect. For example, a shorter welding timein the subsequent current passage is more advantageous in terms ofproductivity. If adaptive control is performed with a shorter weldingtime in the subsequent current passage in a state in which the effect ofcurrent shunting is considerable, however, current control by adaptivecontrol lags behind and a target amount of heat cannot be generated insome cases.

We examined this point more closely, and discovered the following: Aneffective way of achieving a stable heat treatment effect in the casewhere the welding time in the subsequent current passage is short is toperform the subsequent current passage not by adaptive control thatsequentially feeds back the electrical property obtained in thesubsequent current passage, but by constant current control with acurrent determined based on the effect of a disturbance that isestimated based on the electrical property between the electrodes ineach of the main current passage in the test welding and the maincurrent passage in the actual welding.

The present disclosure is based on these discoveries and furtherstudies.

We thus provide:

1. A resistance spot welding method of squeezing, by a pair ofelectrodes, parts to be welded which are a plurality of overlappingmetal sheets, and passing a current while applying an electrode force tojoin the parts to be welded, the resistance spot welding methodcomprising: performing test welding; and performing actual welding afterthe test welding, wherein (a) in the test welding, main current passagefor nugget formation and subsequent current passage for subsequent heattreatment are performed, in the main current passage in the testwelding, a time variation curve of an instantaneous amount of heatgenerated per unit volume and a cumulative amount of heat generated perunit volume that are calculated from an electrical property between theelectrodes in forming an appropriate nugget by performing currentpassage by constant current control are stored, and in the subsequentcurrent passage in the test welding, current passage is performed byconstant current control, and (b) thereafter, in the actual welding,main current passage for nugget formation and subsequent current passagefor subsequent heat treatment are performed, in the main current passagein the actual welding, the time variation curve of the instantaneousamount of heat generated per unit volume and the cumulative amount ofheat generated per unit volume that are stored in the main currentpassage in the test welding are set as a target, and adaptive controlwelding is performed to control a current passage amount according tothe target, and in the subsequent current passage in the actual welding,current passage is performed by constant current control with a currentdetermined based on an electrical property between the electrodes ineach of the main current passage in the test welding and the maincurrent passage in the actual welding.

2. The resistance spot welding method according to 1., wherein

0.8×Itp×(RBtm/RBam)≤Iap≤1.2×Itp×(RBtm/RBam),

where RBtm is an average value of a resistance between the electrodes inthe main current passage in the test welding, RBam is an average valueof a resistance between the electrodes in the main current passage inthe actual welding, Itp is a current in the subsequent current passagein the test welding, and Iap is the current in the subsequent currentpassage in the actual welding.

3. The resistance spot welding method according to 1. or 2., wherein inthe adaptive control welding in the main current passage in the actualwelding, in the case where an amount of time variation of aninstantaneous amount of heat generated per unit volume differs from thetime variation curve of the instantaneous amount of heat generated perunit volume set as the target, the current passage amount is controlledin order to compensate for the difference from the time variation curvewithin a remaining welding time in the main current passage in theactual welding so that a cumulative amount of heat generated per unitvolume in the main current passage in the actual welding matches thecumulative amount of heat generated per unit volume set as the target.

4. The resistance spot welding method according to any one of 1. to 3.,wherein a cooling time is set between the main current passage and thesubsequent current passage in the actual welding, and the number ofrepetitions of a welding interval for the cooling time and thesubsequent current passage after the main current passage is two or moretimes.

5. A weld member production method comprising joining a plurality ofoverlapping metal sheets by the resistance spot welding method accordingto any one of 1. to 4.

Advantageous Effect

It is thus possible to stably ensure a certain nugget diameter andachieve high joint strength in a sheet combination including one or morehigh strength steel sheets, regardless of whether there is adisturbance.

It is also possible to stably ensure a desired nugget diameter byeffectively responding to variations in the disturbance state, even whencontinuously welding parts to be welded which are conveyed one afteranother in real operation such as vehicle manufacturing (even when thedisturbance state varies among welding positions or parts to be welded).This is very advantageous in improving operating efficiency and yieldrate.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1A is a diagram illustrating an example of a current pattern inmain current passage in test welding;

FIG. 1B is a diagram illustrating an example of a current pattern inmain current passage in test welding;

FIG. 1C is a diagram illustrating an example of a current pattern inmain current passage in test welding;

FIG. 1D is a diagram illustrating an example of a current pattern inmain current passage in test welding;

FIG. 1E is a diagram illustrating an example of a current pattern inmain current passage in test welding;

FIG. 1F is a diagram illustrating an example of a current pattern inmain current passage in test welding;

FIG. 2A is a diagram illustrating an example of a current pattern intest welding in the case where main current passage is one-step currentpassage;

FIG. 2B is a diagram illustrating an example of a current pattern intest welding in the case where main current passage is two-step currentpassage;

FIG. 3A is a diagram illustrating an L-shaped tensile test piece used inexamples in the case where the number of overlapping metal sheets is twoand there is no existing weld;

FIG. 3B is a diagram illustrating an L-shaped tensile test piece used inexamples in the case where the number of overlapping metal sheets is twoand there is an existing weld;

FIG. 4A is a diagram illustrating an L-shaped tensile test piece used inexamples in the case where the number of overlapping metal sheets isthree and there is no existing weld; and

FIG. 4B is a diagram illustrating an L-shaped tensile test piece used inexamples in the case where the number of overlapping metal sheets isthree and there is an existing weld.

DETAILED DESCRIPTION

One of the disclosed embodiments relates to a resistance spot weldingmethod of squeezing, by a pair of electrodes, parts to be welded whichare a plurality of overlapping metal sheets, and passing a current whileapplying an electrode force to join the parts to be welded, theresistance spot welding method comprising: performing test welding; andperforming actual welding after the test welding, wherein (a) in thetest welding, main current passage for nugget formation and subsequentcurrent passage for subsequent heat treatment are performed, in the maincurrent passage in the test welding, a time variation curve of aninstantaneous amount of heat generated per unit volume and a cumulativeamount of heat generated per unit volume that are calculated from anelectrical property between the electrodes in forming an appropriatenugget by performing current passage by constant current control arestored, and in the subsequent current passage in the test welding,current passage is performed by constant current control, and (b)thereafter, in the actual welding, main current passage for nuggetformation and subsequent current passage for subsequent heat treatmentare performed, in the main current passage in the actual welding, thetime variation curve of the instantaneous amount of heat generated perunit volume and the cumulative amount of heat generated per unit volumethat are stored in the main current passage in the test welding are setas a target, and adaptive control welding is performed to control acurrent passage amount according to the target, and in the subsequentcurrent passage in the actual welding, current passage is performed byconstant current control with a current determined based on anelectrical property between the electrodes in each of the main currentpassage in the test welding and the main current passage in the actualwelding.

Any welding device that includes a pair of upper and lower electrodesand is capable of freely controlling each of the electrode force and thewelding current during welding may be used in the resistance spotwelding method according to one of the disclosed embodiments. The forcemechanism (air cylinder, servomotor, etc.), the type (stationary, robotgun, etc.), the electrode shape, and the like are not limited. Herein,the “electrical property between the electrodes” means the resistancebetween the electrodes or the voltage between the electrodes.

The test welding and the actual welding in the resistance spot weldingmethod according to one of the disclosed embodiments will be describedbelow.

-   -   Test welding

In the test welding, main current passage for nugget formation andsubsequent current passage for subsequent heat treatment are eachperformed by constant current control.

In the main current passage in the test welding, a time variation curveof an instantaneous amount of heat generated per unit volume and acumulative amount of heat generated per unit volume that are calculatedfrom an electrical property between the electrodes in forming anappropriate nugget by performing current passage by constant currentcontrol are stored.

The test welding may be performed in a state in which there is nodisturbance, or performed in a state in which there is a disturbancesuch as current shunting or a sheet gap (i.e. a state assuming thatthere is a disturbance).

The current pattern in the main current passage in the test welding maybe a current pattern of a constant current throughout the currentpassage. The current pattern may be a current pattern divided into twoor more steps each of which has a constant current, as illustrated inFIGS. 1A and 1B. The current pattern may be a current pattern of two ormore steps with a cooling time being provided therebetween, asillustrated in FIG. 1C. The current pattern may be a current pattern inslope form, as illustrated in FIGS. 1D to 1F. The current pattern may beany combination of these patterns.

Herein, the term “constant current control” includes not only a currentpattern of a constant current throughout the current passage, but alsothe current patterns illustrated in FIGS. 1A to 1F, and any currentpatterns combining these current patterns. The same applies to constantcurrent control performed in the subsequent current passage in each ofthe test welding and the actual welding.

A preferable range of the current in the main current passage in thetest welding varies depending on which sheet combination is used as theparts to be welded. For example, in the case of using a sheetcombination of steel sheets of 980 MPa-grade in tensile strength (TS)and 1.2 mm to 1.6 mm in sheet thickness, the current in the main currentpassage in the test welding is preferably in a range of 3.0 kA to 12.0kA.

The total welding time in the main current passage in the test weldingexcluding the cooling time is preferably in a range of 60 ms to 1000 ms.

In the subsequent current passage in the test welding, current passageis performed by constant current control to carry out subsequent heattreatment.

A time variation curve of an instantaneous amount of heat generated perunit volume and a cumulative amount of heat generated per unit volume inthe subsequent current passage in the test welding may or may not bestored.

A preferable range of the current in the subsequent current passage inthe test welding varies depending on which sheet combination is used asthe parts to be welded. For example, in the case of using a sheetcombination of steel sheets of 980 MPa-grade in tensile strength (TS)and 1.2 mm to 1.6 mm in sheet thickness, the current in the subsequentcurrent passage in the test welding is preferably in a range of 3.0 kAto 15.0 kA.

The welding time per one subsequent current passage in the test weldingis preferably in a range of 10 ms to 200 ms.

Actual Welding

After the test welding, the actual welding is performed.

In the main current passage in the actual welding, the time variationcurve of the instantaneous amount of heat generated per unit volume andthe cumulative amount of heat generated per unit volume that are storedin the main current passage in the test welding are set as a target, andadaptive control welding is performed to control a current passageamount according to the target.

For example, in the adaptive control welding in the main current passagein the actual welding, welding is performed with the time variationcurve of the instantaneous amount of heat generated per unit volume andthe cumulative amount of heat generated per unit volume that are storedin the main current passage in the test welding being set as the target.If the amount of time variation of the instantaneous amount of heatgenerated per unit volume follows the time variation curve, the weldingis continued without change and completed. If the amount of timevariation of the instantaneous amount of heat generated per unit volumediffers from the time variation curve, the current passage amount iscontrolled in order to compensate for the difference within a remainingwelding time in the main current passage in the actual welding so thatthe cumulative amount of heat generated per unit volume in the maincurrent passage in the actual welding matches the cumulative amount ofheat generated per unit volume set as the target.

A method of calculating the amount of heat generated is not limited. PTL5 describes an example of the method, which may be used herein. Thefollowing is the procedure of calculating the amount q of heat generatedper unit volume and per unit time and the cumulative amount Q of heatgenerated per unit volume according to this method.

Let t be the total thickness of the parts to be welded, r be theelectrical resistivity of the parts to be welded, V be the voltagebetween the electrodes, I be the welding current, and S be the contactarea of the electrodes and the parts to be welded. In this case, thewelding current passes through a columnar portion whose cross-sectionalarea is S and thickness is t, to generate heat by resistance. The amountq of heat generated per unit volume and per unit time in the columnarportion is given by the following Equation (1):

q=(V·I)/(S·t)  (1).

The electrical resistance R of the columnar portion is given by thefollowing Equation (2):

R=(r·t)/S  (2).

Solving Equation (2) for S and substituting the solution into Equation(1) yields the amount q of heat generated as indicated by the followingEquation (3):

q=(V·I·R)/(r·t ²)=(V ²)/(r·t ²)  (3).

As is clear from Equation (3), the amount q of heat generated per unitvolume and per unit time can be calculated from the voltage V betweenthe electrodes, the total thickness t of the parts to be welded, and theelectrical resistivity r of the parts to be welded, and is not affectedby the contact area S of the electrodes and the parts to be welded.Although the amount of heat generated is calculated from the voltage Vbetween the electrodes in Equation (3), the amount q of heat generatedmay be calculated from the interelectrode current I. The contact area Sof the electrodes and the parts to be welded need not be used in thiscase, either. By cumulating the amount q of heat generated per unitvolume and per unit time for the welding time, the cumulative amount Qof heat generated per unit volume for the welding is obtained. As isclear from Equation (3), the cumulative amount Q of heat generated perunit volume can also be calculated without using the contact area S ofthe electrodes and the parts to be welded.

Although the above describes the case of calculating the cumulativeamount Q of heat generated by the method described in PTL 5, thecumulative amount Q may be calculated by any other method.

In the subsequent current passage in the actual welding, it is importantto perform current passage by constant current control with a currentdetermined based on the electrical property between the electrodes ineach of the main current passage in the test welding and the maincurrent passage in the actual welding.

As mentioned earlier, if the subsequent current passage is performed byadaptive control in the presence of a disturbance, the current densitydistribution of the weld and thus the heat generation pattern change dueto the disturbance in some cases, making it impossible to achieve thepredetermined heat treatment effect. In particular, if adaptive controlis performed with a shorter welding time in the subsequent currentpassage in a state in which the effect of current shunting isconsiderable, current control by adaptive control lags behind and atarget amount of heat cannot be generated in some cases.

If the subsequent current passage in the actual welding is performed byconstant current control with the current determined based on theelectrical property between the electrodes in each of the main currentpassage in the test welding and the main current passage in the actualwelding, on the other hand, the predetermined heat treatment effect canbe achieved even in the case where there is a disturbance and thewelding time is short.

It is therefore important to, in the subsequent current passage in theactual welding, perform current passage by constant current control withthe current determined based on the electrical property between theelectrodes in each of the main current passage in the test welding andthe main current passage in the actual welding.

For example, the predetermined heat treatment effect can be achievedeven in the case where there is a disturbance and the welding time isshort, by performing the subsequent current passage in the actualwelding by constant current control under the condition that Iapsatisfies

0.8×Itp×(RBtm/RBam)≤Iap≤1.2×Itp×(RBtm/RBam)

where RBtm is an average value of the resistance between the electrodesin the main current passage in the test welding, RBam is an averagevalue of the resistance between the electrodes in the main currentpassage in the actual welding, Itp is the current in the subsequentcurrent passage in the test welding, and Iap is the current in thesubsequent current passage in the actual welding.

This is because, from the ratio (RBtm/RBam) between the resistancebetween the electrodes in the main current passage in the test weldingand the resistance between the electrodes in the main current passage inthe actual welding, a current necessary for the subsequent currentpassage can be estimated with the effect of a disturbance being takeninto consideration.

More preferably, Iap satisfies

0.9×Itp×(RBtm/RBam)≤Iap≤1.1×Itp×(RBtm/RBam).

In the main current passage in each of the test welding and the actualwelding, in the case where a cooling time is provided during the currentpassage as illustrated in FIG. 1C, a time average value of theresistance between the electrodes during the current passage excludingthe cooling time is taken to be the average value of the resistancebetween the electrodes.

In detail, a value obtained by dividing a time integration value of theresistance between the electrodes in the main current passage by thetotal welding time in the main current passage excluding the coolingtime is taken to be the average value of the resistance between theelectrodes in the main current passage. The same applies to thesubsequent current passage.

Instead of the average value of the resistance between the electrodes inthe main current passage in each of the test welding and the actualwelding, an average value of the current in the main current passage ineach of the test welding and the actual welding may be used.

In this case, the predetermined heat treatment effect can be achievedeven in the case where there is a disturbance and the welding time isshort, by performing the subsequent current passage in the actualwelding by constant current control under the condition that lapsatisfies

0.8×Itp×(IBam/IBtm)≤Iap≤1.2×Itp×(IBam/IBtm),

where IBtm is an average value of the current in the main currentpassage in the test welding, and IBam is an average value of the currentin the main current passage in the actual welding.

More preferably, lap satisfies

0.9×Itp×(IBam/IBtm)≤Iap≤1.1×Itp×(IBam/IBtm).

In the main current passage in each of the test welding and the actualwelding, in the case where a cooling time is provided during the currentpassage as illustrated in FIG. 1C, a time average value of the currentduring the current passage excluding the cooling time is taken to be theaverage value of the current.

In detail, a value obtained by dividing a time integration value of thecurrent in the main current passage by the total welding time in themain current passage excluding the cooling time is taken to be theaverage value of the current in the main current passage. The sameapplies to the subsequent current passage.

The welding time per one subsequent current passage in the actualwelding is preferably in a range of 10 ms to 200 ms.

In each of the test welding and the actual welding, a cooling time maybe set between the main current passage and the subsequent currentpassage. The cooling time is preferably in a range of 20 ms to 2000 ms.

In each of the test welding and the actual welding, the number of timesa welding interval for the cooling time and the subsequent currentpassage are performed after the main current passage may be two or moretimes, as illustrated in FIGS. 2A and 2B (the number of times thewelding interval for the cooling time and the subsequent current passageare performed after the main current passage is defined as the number ofrepetitions N). In this way, the predetermined heat treatment effect canbe achieved more favorably.

Even if heat is generated excessively in the first subsequent currentpassage and remelting occurs, by performing heat treatment in the secondsubsequent current passage, the effect of improving joint strength canbe achieved. No upper limit is placed on the number of repetitions, butthe upper limit of the number of repetitions is preferably about 10. Thewelding time, the cooling time, and the current may be different eachtime.

In the case where the number of repetitions of the welding interval forthe cooling time and the subsequent current passage after the maincurrent passage is two or more times, the current Itp in the subsequentcurrent passage in the test welding and the current Iap in thesubsequent current passage in the actual welding are each a valueobtained by dividing a time integration value of the current in thesubsequent current passage by the total welding time in the subsequentcurrent passage excluding the cooling time.

The conditions in the actual welding other than those described abovemay be basically the same as the conditions in the test welding.

The parts to be welded or the sheet combination used is not limited. Theresistance spot welding method may be used for steel sheets and coatedsteel sheets having various strengths from mild steel to ultra hightensile strength steel. The resistance spot welding method may also beused for a sheet combination of three or more overlapping steel sheets,and is particularly advantageous in the case where one or more steelsheets of the sheet combination has a tensile strength of 590 MPa ormore.

In each of the test welding and the actual welding, the electrode forcein the current passage may be constant, or be changed as appropriate. Apreferable range of the electrode force varies depending on which sheetcombination is used as the parts to be welded. For example, in the caseof using a sheet combination of two overlapping steel sheets of 980MPa-grade in tensile strength (TS) and 1.2 mm to 1.6 mm in sheetthickness, the electrode force is preferably in a range of 1.5 kN to10.0 kN.

By joining a plurality of overlapping metal sheets by the resistancespot welding method described above, various high-strength weld members,in particular weld members of automotive parts and the like, areproduced while stably ensuring a desired nugget diameter by effectivelyresponding to variations in the disturbance state.

EXAMPLES

The presently disclosed techniques will be described below, by way ofexamples. The conditions in the examples are one example of conditionsemployed to determine the operability and effects of the presentlydisclosed techniques, and the present disclosure is not limited to suchexample of conditions. Various conditions can be used in the presentdisclosure as long as the object of the present disclosure is fulfilled,without departing from the scope of the present disclosure.

Test welding was performed under the conditions listed in Table 2 foreach sheet combination of two or three overlapping metal sheets listedin Table 1, and then actual welding was performed under the conditionslisted in Table 3 for the same sheet combination, to produce a weldjoint (L-shaped tensile test piece).

FIGS. 2A and 2B illustrate current patterns in the test welding. FIG. 2Aillustrates a current pattern in the case where the main current passageis one-step current passage, and FIG. 2B illustrates a current patternin the case where the main current passage is two-step current passage.

The test welding was performed in a state in which there was nodisturbance, as illustrated in FIGS. 3A and 4A. The actual welding wasperformed in a state in which there was no disturbance as in the testwelding, and in a state in which there was a disturbance as illustratedin FIGS. 3B and 4B.

FIG. 3A illustrates a state in which the number of overlapping metalsheets is two and there is no existing weld. FIG. 3B illustrates a statein which the number of overlapping metal sheets is two and there is anexisting weld. The welding spot spacing L (center-to-center spacing)between the existing weld and the welding point (current welding point)was varied.

FIG. 4A illustrates a state in which the number of overlapping metalsheets is three and there is no existing weld. FIG. 4B illustrates astate in which the number of overlapping metal sheets is three and thereis an existing weld.

The “welding time in subsequent current passage” in each of the testwelding condition in Table 2 and the actual welding condition in Table 3is the welding time per one subsequent current passage. The coolingtime, the current in subsequent current passage, and the welding time insubsequent current passage in each of the test welding condition inTable 2 and the actual welding condition in Table 3 were the same foreach subsequent current passage.

In the “control method of main current passage” in the actual weldingcondition in Table 3, “constant current control” indicates constantcurrent control performed under the same condition as in the testwelding. In the “current determination method in constant currentcontrol” in Table 3,

“Formula (A)”, “Formula (B)”, and “Formula (C)” respectively indicatedetermining the current Iap in the subsequent current passage in theactual welding according to the following Formulas (A), (B), and (C)that are each within the foregoing range of 0.8×Itp×(RBtm/RBam) Iap1.2×Itp×(RBtm/RBam):

Iap=0.8×Itp×(RBtm/RBam)  Formula (A):

Iap=1.0×Itp×(RBtm/RBam)  Formula (B):

Iap=1.2×Itp×(RBtm/RBam).  Formula (C):

In the case where actual welding was performed in a state in which therewas an existing weld, the below-described tensile test was conductedafter cutting off the part of the existing weld from the L-shapedtensile test piece.

An inverter DC resistance spot welder was used as the welder, andchromium copper electrodes with 6 mm face diameter DR-shaped tips wereused as the electrodes.

Each obtained L-shaped tensile test piece was used to conduct a tensiletest at a tension rate (in longitudinal direction) of 10 mm/min, and thejoint strength (L-shape tensile strength (LTS)) was measured. Based onwhether expulsion occurred in welding and the joint strength, evaluationwas performed in the following three levels:

-   -   A: LTS was 2.0 kN or more regardless of welding spot spacing L,        and no expulsion occurred.    -   B: LTS was 2.0 kN or more when there was no existing weld or        when welding spot spacing L≥10 mm, LTS was less than 2.0 kN when        welding spot spacing L<10 mm, and no expulsion occurred.    -   F: LTS was less than 2.0 kN when there was no existing weld or        when welding spot spacing L≥10 mm, or expulsion occurred.

TABLE 1 Sheet combination Intermediate ID Upper sheet sheet Lower sheetA1 1180 MPa-grade 1180 MPa-grade cold-rolled steel cold-rolled steelsheet sheet (sheet thickness: (sheet thickness: 1.6 mm) 1.6 mm) A2 980MPa-grade 980 MPa-grade cold-rolled steel cold-rolled steel sheet sheet(sheet thickness: (sheet thickness: 1.6 mm) 1.6 mm) A3 980 MPa-grade1470 MPa-grade cold-rolled steel cold-rolled steel sheet sheet (sheetthickness: (sheet thickness: 1.4 mm) 1.2 mm) A4 980 MPa-grade 1470MPa-grade GA GA steel sheet steel sheet (sheet thickness: (sheetthickness: 1.2 mm) 2.0 mm) A5 1470 MPa-grade 1470 MPa-grade GA GA steelsheet steel sheet (sheet thickness: (sheet thickness: 1.4 mm) 1.4 mm) A61800 MPa-grade 980 MPa-grade GA noncoated hot steel sheet stamp steelsheet (sheet thickness: (sheet thickness: 1.2 mm) 1.4 mm) A7 1800MPa-grade 1180 MPa-grade Zn—Ni-coated cold-rolled steel sheet hot stampsteel (sheet thickness: sheet (sheet 1.4 mm) thickness: 1.6 mm) B1 270MPa-grade 1470 MPa-grade 1470 MPa-grade GA steel sheet GA steel sheet GAsteel sheet (sheet thickness: (sheet thickness: (sheet thickness: 0.7mm) 1.4 mm) 1.4 mm)

TABLE 2 Test welding condition Main current passage Average ofresistance between Subsequent current passage Current Welding CurrentWelding Average time in Current Welding 1 in time 1 2 in time 2 ofcurrent electrodes in time in Elec- main in main main in main in main inmain subsequent subsequent Sheet trode current current current currentcurrent current Cooling current current Number of combi- force passagepassage passage passage passage passage time passage passage repetitionsnation F I_(tm)1 t_(tm)1 I_(tm)2 t_(tm)2 Ibtm RB_(tm) t_(c) I_(tp)t_(tp) N No. ID (kN) (kA) (ms) (kA) (ms) (kA) (μΩ) (ms) (kA) (ms)(times) Remarks 1  1-1 A1 4.5 6.0 320 — — 6.0 200 160 9.5 40 2 Ex.  1-2A1 4.5 6.0 320 — — 6.0 200 160 9.5 40 2  1-3 A1 4.5 6.0 320 — — 6.0 200160 9.5 40 2 2  2-1 A1 4.5 6.0 320 — — 6.0 200 160 9.5 40 2 Ex.  2-2 A14.5 6.0 320 — — 6.0 200 160 9.5 40 2  2-3 A1 4.5 6.0 320 — — 6.0 200 1609.5 40 2 3  3-1 A1 4.5 6.0 320 — — 6.0 200 80 11.5 40 1 Ex.  3-2 A1 4.56.0 320 — — 6.0 200 80 11.5 40 1  3-3 A1 4.5 6.0 320 — — 6.0 200 80 11.540 1 4  4-1 A1 4.5 6.0 320 — — 6.0 200 80 6.0 40 1 Comp.  4-2 A1 4.5 6.0320 — — 6.0 200 80 6.0 40 1 Ex.  4-3 A1 4.5 6.0 320 — — 6.0 200 80 6.040 1 5  5-1 A1 4.5 6.0 320 — — 6.0 200 160 6.0 40 1 Comp.  5-2 A1 4.56.0 320 — — 6.0 200 160 6.0 40 1 Ex.  5-3 A1 4.5 6.0 320 — — 6.0 200 1606.0 40 1 6  6-1 A1 4.5 4.0 100 6.0 240 5.4 220 160 9.5 40 2 Ex.  6-2 A14.5 4.0 100 6.0 240 5.4 220 160 9.5 40 2  6-3 A1 4.5 4.0 100 6.0 240 5.4220 160 9.5 40 2 7  7-1 A1 4.5 6.0 320 — — 6.0 200 160 6.0 40 1 Comp. 7-2 A1 4.5 6.0 320 — — 6.0 200 160 6.0 40 1 Ex.  7-3 A1 4.5 6.0 320 — —6.0 200 160 6.0 40 1 8  8-1 A1 4.5 6.0 320 — — 6.0 200 160 10.0 60 2 Ex. 8-2 A1 4.5 6.0 320 — — 6.0 200 160 10.0 60 2  8-3 A1 4.5 6.0 320 — —6.0 200 160 10.0 60 2 9  9-1 A1 4.5 6.0 320 — — 6.0 200 160 9.5 40 10Ex.  9-2 A1 4.5 6.0 320 — — 6.0 200 160 9.5 40 10  9-3 A1 4.5 6.0 320 —— 6.0 200 160 9.5 40 10 10 10-1 A2 3.5 5.5 280 — — 5.5 210 80 8.5 40 2Ex. 10-2 A2 3.5 5.5 280 — — 5.5 210 80 8.5 40 2 10-3 A2 3.5 5.5 280 — —5.5 210 80 8.5 40 2 11 11-1 A3 5.5 6.2 260 — — 6.2 180 80 10.0 40 2 Ex.11-2 A3 5.5 6.2 260 — — 6.2 180 80 10.0 40 2 11-3 A3 5.5 6.2 260 — — 6.2180 80 10.0 40 2 12 12-1 A4 6.0 6.5 300 — — 6.5 190 80 10.0 40 2 Ex.12-2 A4 6.0 6.5 300 — — 6.5 190 80 10.0 40 2 12-3 A4 6.0 6.5 300 — — 6.5190 80 10.0 40 2 13 13-1 A5 5.0 7.0 280 — — 7.0 160 80 10.5 40 2 Ex.13-2 A5 5.0 7.0 280 — — 7.0 160 80 10.5 40 2 13-3 A5 5.0 7.0 280 — — 7.0160 80 10.5 40 2 14 14-1 A6 5.0 6.0 400 — — 6.0 190 80 9.0 40 2 Ex. 14-2A6 5.0 6.0 400 — — 6.0 190 80 9.0 40 2 14-3 A6 5.0 6.0 400 — — 6.0 19080 9.0 40 2 15 15-1 B1 5.0 7.0 320 — — 9.0 200 80 9.0 40 2 Ex. 15-2 B15.0 7.0 320 — — 9.0 200 80 9.0 40 2 15-3 B1 5.0 7.0 320 — — 9.0 200 809.0 40 2 16 16-1 B1 5.0 4.0 160 7.5 200 5.9 180 80 8.5 40 2 Ex. 16-2 B15.0 4.0 160 7.5 200 5.9 180 80 8.5 40 2 16-3 B1 5.0 4.0 160 7.5 200 5.9180 80 8.5 40 2 17 17-1 A7 5.5 5.0 240 7.2 240 6.1 190 100 8.5 40 2 Ex.17-2 A7 5.5 5.0 240 7.2 240 6.1 190 100 8.5 40 2 17-3 A7 5.5 5.0 240 7.2240 6.1 190 100 8.5 40 2

TABLE 3 Actual welding condition Main current passage Average ofresistance Subsequent current passage Average between Current CurrentWelding of current electrodes determi- Calcu- Calcu- Calcu- in subse-time Number Elec- Control in main in main nation lation lation lationquent in subse- of Sheet trode method current current Control methodmethod in value value value current quent repe- combi- Welding force ofmain passage passage of subsequent constant of of of Cooling passagecurrent titions nation spot spacing F current IB_(am) RB_(am) currentcurrent Formula Formula Formula time I_(ap) passage N LTS Expul- Eval-No. ID L (kN) passage (kA) (μΩ) passage control (A) (B) (C) (ms) (kA)(ms) (times) (kN) sion uation Remarks 1  1-1 A1 No existing 4.5 Adaptive6.0 200 Constant current Formula (B) 7.6 9.5 11.4 160 9.5 40 2 3.3 NoneA Ex. weld control control  1-2 A1 10 mm 4.5 Adaptive 6.4 192 Constantcurrent Formula (B) 7.9 9.9 11.9 160 9.9 40 2 3.1 None control control 1-3 A1 7 mm 4.5 Adaptive 6.7 185 Constant current Formula (B) 8.2 10.312.3 160 10.3 40 2 3.6 None control control 2  2-1 A1 No existing 4.5Adaptive 6.0 200 Constant current Formula (C) 7.6 9.5 11.4 160 11.4 40 23.3 None A Ex. weld control control  2-2 A1 10 mm 4.5 Adaptive 6.4 192Constant current Formula (C) 7.9 9.9 11.9 160 11.9 40 2 3.1 None controlcontrol  2-3 A1 7 mm 4.5 Adaptive 6.7 185 Constant current Formula (C)8.2 10.3 12.3 160 12.3 40 2 3.2 None control control 3  3-1 A1 Noexisting 4.5 Adaptive 6.0 200 Constant current Formula (B) 9.2 11.5 13.880 11.5 40 1 2.8 None B Ex. weld control control  3-2 A1 10 mm 4.5Adaptive 6.4 192 Constant current Formula (B) 9.6 12.0 14.4 80 12.0 40 12.9 None control control  3-3 A1 7 mm 4.5 Adaptive 6.7 185 Constantcurrent Formula (B) 9.9 12.4 14.9 80 12.4 40 1 1.7 None control control4  4-1 A1 No existing 4.5 Adaptive 6.0 200 Adaptive control — — — — 80 —40 1 2.8 None F Comp. weld control Ex.  4-2 A1 10 mm 4.5 Adaptive 6.4192 Adaptive control — — — — 80 — 40 1 1.8 None control  4-3 A1 7 mm 4.5Adaptive 6.7 185 Adaptive control — — — — 80 — 40 1 1.7 None control 5 5-1 A1 No existing 4.5 Adaptive 6.0 200 Constant current Same as test —— — 160 6.0 40 1 2.6 None F Comp. weld control control welding Ex.  5-2A1 10 mm 4.5 Adaptive 6.4 192 Constant current Same as test — — — 1606.0 40 1 1.8 None control control welding  5-3 A1 7 mm 4.5 Adaptive 6.7186 Constant current Same as test — — — 160 6.0 40 1 1.6 None controlcontrol welding 6  6-1 A1 No existing 4.5 Adaptive 5.4 220 Constantcurrent Formula (B) 7.6 9.5 11.4 160 9.5 40 2 3.4 None A Ex. weldcontrol control  6-2 A1 10 mm 4.5 Adaptive 5.8 210 Constant currentFormula (B) 8.0 10.0 11.9 160 10.0 40 2 3.5 None control control  6-3 A17 mm 4.5 Adaptive 6.2 200 Constant current Formda (B) 8.4 10.5 12.5 16010.5 40 2 3.3 None control control 7  7-1 A1 No existing 4.5 Constant6.0 200 Constant current Same as test 4.8 6.0 7.2 160 6.0 40 1 2.6 NoneF Comp. weld current control welding Ex. control  7-2 A1 10 mm 4.5Constant 6.0 192 Constant current Same as test 5.0 6.3 7.5 160 6.0 40 11.6 None current control welding control  7-3 A1 7 mm 4.5 Constant 6.0185 Constant current Same as test 5.2 6.5 7.8 160 6.0 40 1 1.4 Nonecurrent control welding control 8  8-1 A1 No existing 4.5 Adaptive 6.0200 Constant current Formula (A) 8.0 10.0 12.0 160 8.0 60 2 3.4 None AEx. weld control control  8-2 A1 10 mm 4.5 Adaptive 6.4 192 Constantcurrent Formula (A) 8.3 10.4 12.5 160 8.3 60 2 3.2 None control control 8-3 A1 7 mm 4.5 Adaptive 6.7 185 Constant current Formula (A) 8.6 10.813.0 160 8.6 60 2 3.5 None control control 9  9-1 A1 No existing 4.5Adaptive 6.0 200 Constant current Formula (B) 7.6 9.5 11.4 160 9.5 40 103.2 None A Ex. weld control control  9-2 A1 10 mm 4.5 Adaptive 6.4 192Constant current Formula (B) 7.9 9.9 11.9 160 9.9 40 10 3.5 None controlcontrol  9-3 A1 7 mm 4.5 Adaptive 6.7 185 Constant current Formula (B)8.2 10.3 12.3 160 10.3 40 10 3.1 None control control 10 10-1 A2 Noexisting 3.5 Adaptive 6.0 210 Constant current Formula (B) 6.8 8.5 10.280 8.5 40 2 3.1 None A Ex. weld control control 10-2 A2 10 mm 3.5Adaptive 6.4 190 Constant current Formula (B) 7.5 9.4 11.3 80 9.4 40 23.3 None control control 10-3 A2 7 mm 3.5 Adaptive 6.7 180 Constantcurrent Formula (B) 7.9 9.9 11.9 80 9.9 40 2 3.1 None control control 1111-1 A3 No existing 5.5 Adaptive 6.0 180 Constant current Formula (B)8.0 10.0 12.0 80 10.0 40 2 2.9 None A Ex. weld control control 11-2 A310 mm 5.5 Adaptive 6.4 172 Constant current Formula (B) 8.4 10.5 12.6 8010.5 40 2 2.8 None control control 11-3 A3 7 mm 5.5 Adaptive 6.7 165Constant current Formula (B) 8.7 10.9 13.1 80 10.9 40 2 2.7 None controlcontrol 12 12-1 A4 No existing 6.0 Adaptive 6.0 210 Constant currentFormula (B) 7.2 9.0 10.9 80 9.0 40 2 3.5 None A Ex. weld control control12-2 A4 10 mm 6.0 Adaptive 6.4 190 Constant current Formula (B) 8.0 10.012.0 80 10.0 40 2 3.3 None control control 12-3 A4 7 mm 6.0 Adaptive 6.7180 Constant current Formula (B) 8.4 10.6 12.7 80 10.6 40 2 3.2 Nonecontrol control 13 13-1 A5 No existing 5.0 Adaptive 6.0 160 Constantcurrent Formula (B) 8.4 10.5 12.6 80 10.5 40 2 3.2 None A Ex. weldcontrol control 13-2 A5 10 mm 5.0 Adaptive 6.4 155 Constant currentFormula (B) 8.7 10.8 13.0 80 10.8 40 2 3.4 None control control 13-3 A57 mm 5.0 Adaptive 6.7 149 Constant current Formula (B) 9.0 11.3 13.5 8011.3 40 2 3.4 None control control 14 14-1 A6 No existing 5.0 Adaptive6.0 190 Constant current Formula (B) 7.2 9.0 10.8 80 9.0 40 2 2.3 None AEx. weld control control 14-2 A6 10 mm 5.0 Adaptive 6.4 181 Constantcurrent Formula (B) 7.6 9.4 11.3 80 9.4 40 2 2.5 None control control14-3 A6 7 mm 5.0 Adaptive 6.7 174 Constant current Formula (B) 7.9 9.811.8 80 9.8 40 2 2.4 None control control 15 15-1 B1 No existing 5.0Adaptive 6.0 200 Constant current Formula (B) 7.2 9.0 10.8 80 9.0 40 23.3 None A Ex. weld control control 15-2 B1 10 mm 5.0 Adaptive 6.4 192Constant current Formula (B) 7.5 9.4 11.3 80 9.4 40 2 3.5 None controlcontrol 15-3 B1 7 mm 5.0 Adaptive 6.7 185 Constant current Formula (B)7.8 9.7 11.7 80 9.7 40 2 3.2 None control control 16 16-1 B1 No existing5.0 Adaptive 6.0 180 Constant current Formula (B) 6.8 8.5 10.2 80 8.5 402 2.6 None A Ex. weld control control 16-2 B1 10 mm 5.0 Adaptive 6.4 171Constant current Formula (B) 7.2 8.9 10.7 80 8.9 40 2 2.8 None controlcontrol 16-3 B1 7 mm 5.0 Adaptive 6.7 162 Constant current Formula (B)7.6 9.4 11.3 80 9.4 40 2 2.3 None control control 17 17-1 A7 No existing5.5 Adaptive 6.0 192 Constant current Formula (B) 6.7 8.4 10.1 100 8.440 2 2.5 None A Ex. weld control control 17-2 A7 10 mm 5.5 Adaptive 6.3182 Constant current Formula (B) 7.1 8.9 10.6 100 8.9 40 2 2.3 Nonecontrol control 17-3 A7 7 mm 5.5 Adaptive 6.8 175 Constant currentFormula (B) 7.4 9.2 11.1 100 9.2 40 2 2.2 None control control A: LTSwas 2.0 kN or more regardless of welding spot spacing L, and noexplusion occurred. B: LTS was 2.0 kN or more when there was no existingweld or when welding spot spacing L ≥ 10 mm, LTS was less than 2.0 kNwhen welding spot spacing L < 10 mm, and no explusion occurred. F: LTSwas less than 2.0 kN when there was no existing weld or when weldingspot spacing L ≥ 10 mm, or explusion occurred.

As can be seen in Table 3, all Examples (Ex.) were evaluated as A or B.In particular, all Examples in which the number of repetitions of thewelding interval for the cooling time and the subsequent current passageafter the main current passage was two or more times were evaluated asA.

All Comparative Examples (Comp. Ex.) not satisfying the appropriateconditions according to the present disclosure were evaluated as F, andcould not obtain sufficient joint strength.

1. A resistance spot welding method of squeezing, by a pair ofelectrodes, parts to be welded which are a plurality of overlappingmetal sheets, and passing a current while applying an electrode force tojoin the parts to be welded, the resistance spot welding methodcomprising: performing test welding; and performing actual welding afterthe test welding, wherein (a) in the test welding, main current passagefor nugget formation and subsequent current passage for subsequent heattreatment are performed, in the main current passage in the testwelding, a time variation curve of an instantaneous amount of heatgenerated per unit volume and a cumulative amount of heat generated perunit volume that are calculated from an electrical property between theelectrodes in forming an appropriate nugget by performing currentpassage by constant current control are stored, and in the subsequentcurrent passage in the test welding, current passage is performed byconstant current control, and (b) thereafter, in the actual welding,main current passage for nugget formation and subsequent current passagefor subsequent heat treatment are performed, in the main current passagein the actual welding, the time variation curve of the instantaneousamount of heat generated per unit volume and the cumulative amount ofheat generated per unit volume that are stored in the main currentpassage in the test welding are set as a target, and adaptive controlwelding is performed to control a current passage amount according tothe target, and in the subsequent current passage in the actual welding,current passage is performed by constant current control with a currentdetermined based on an electrical property between the electrodes ineach of the main current passage in the test welding and the maincurrent passage in the actual welding.
 2. The resistance spot weldingmethod according to claim 1, wherein0.8×Itp×(RBtm/RBam)≤Iap≤1.2×Itp×(RBtm/RBam), where RBtm is an averagevalue of a resistance between the electrodes in the main current passagein the test welding, RBam is an average value of a resistance betweenthe electrodes in the main current passage in the actual welding, Itp isa current in the subsequent current passage in the test welding, and Iapis the current in the subsequent current passage in the actual welding.3. The resistance spot welding method according to claim 1, wherein inthe adaptive control welding in the main current passage in the actualwelding, in the case where an amount of time variation of aninstantaneous amount of heat generated per unit volume differs from thetime variation curve of the instantaneous amount of heat generated perunit volume set as the target, the current passage amount is controlledin order to compensate for the difference from the time variation curvewithin a remaining welding time in the main current passage in theactual welding so that a cumulative amount of heat generated per unitvolume in the main current passage in the actual welding matches thecumulative amount of heat generated per unit volume set as the target.4. The resistance spot welding method according to claim 1, wherein acooling time is set between the main current passage and the subsequentcurrent passage in the actual welding, and the number of repetitions ofa welding interval for the cooling time and the subsequent currentpassage after the main current passage is two or more times.
 5. A weldmember production method comprising joining a plurality of overlappingmetal sheets by the resistance spot welding method according to claim 1.6. The resistance spot welding method according to claim 2, wherein inthe adaptive control welding in the main current passage in the actualwelding, in the case where an amount of time variation of aninstantaneous amount of heat generated per unit volume differs from thetime variation curve of the instantaneous amount of heat generated perunit volume set as the target, the current passage amount is controlledin order to compensate for the difference from the time variation curvewithin a remaining welding time in the main current passage in theactual welding so that a cumulative amount of heat generated per unitvolume in the main current passage in the actual welding matches thecumulative amount of heat generated per unit volume set as the target.7. The resistance spot welding method according to claim 2, wherein acooling time is set between the main current passage and the subsequentcurrent passage in the actual welding, and the number of repetitions ofa welding interval for the cooling time and the subsequent currentpassage after the main current passage is two or more times.
 8. Theresistance spot welding method according to claim 3, wherein a coolingtime is set between the main current passage and the subsequent currentpassage in the actual welding, and the number of repetitions of awelding interval for the cooling time and the subsequent current passageafter the main current passage is two or more times.
 9. The resistancespot welding method according to claim 6, wherein a cooling time is setbetween the main current passage and the subsequent current passage inthe actual welding, and the number of repetitions of a welding intervalfor the cooling time and the subsequent current passage after the maincurrent passage is two or more times.
 10. A weld member productionmethod comprising joining a plurality of overlapping metal sheets by theresistance spot welding method according to claim
 2. 11. A weld memberproduction method comprising joining a plurality of overlapping metalsheets by the resistance spot welding method according to claim
 3. 12. Aweld member production method comprising joining a plurality ofoverlapping metal sheets by the resistance spot welding method accordingto claim
 4. 13. A weld member production method comprising joining aplurality of overlapping metal sheets by the resistance spot weldingmethod according to claim
 6. 14. A weld member production methodcomprising joining a plurality of overlapping metal sheets by theresistance spot welding method according to claim
 7. 15. A weld memberproduction method comprising joining a plurality of overlapping metalsheets by the resistance spot welding method according to claim
 8. 16. Aweld member production method comprising joining a plurality ofoverlapping metal sheets by the resistance spot welding method accordingto claim 9.